In motors used in robot joints and so on, a multi-turn absolute encoder equipped with a drum on which a magnetic scale is formed, a magnetic sensor element, a counter, and so on may be provided to make it unnecessary to return to a square point when starting up. In this case, the multi-turn absolute encoder is backed up by a battery to detect a multi-turn position even during a stop period and to hold a detection result (for example, refer to Patent Literature 1).